But upon thinking about it, I think such "bait and switch" concepts are not a good idea, in general. I would not like my character to be changed at the opening game session with no warning or no action by me.
Well, that's why you need to separate the change in the character and the change in the setting. This is especially true since you haven't told us what mutations happened. Some may be more or less acceptable than others. Laser eyes or gills is not the same as say growing a second head, or losing your legs.
This is why your "in general" isn't quite as good as you might think. There are so many ways to do it, there's just no way to be sure of it. Now I would agree for a cautious thoughtful approach in general, but I wouldn't say it's not a good idea, but rather a risky one.
Which is often true of many good ideas.
As a general rule, I would not like to be told we're going to play a Greyhawk campaign, and create my Greyhawk-style character, and then discover in the opening of the first game session that we're in Darksun and that my elf is now an orc. Sure, it might turn out to be fun, but it's poor form.
OTOH, what if your DM made a rough description, and told you that you were going on a boat ride, and gave you a rough idea of where you expected to land...but then you landed on Gilligan's Island, or the Lost Island, or the Land of the Lost....
And of course, there's the classic approach used in the Dungeons and Dragons cartoon...bunch of random high school types put in a world they never expected!
DM's are not gods, they do not always know what is best for the game.
Not always? Sure. Nobody is always right. Including the players too. Sometimes the DM might do something they don't want, or expect, and this may or may not be bad.
Wow, I couldn't disagree more. We have different philosophies on this.
I don't think you understand the philosophies at hand very accurately, as what he's saying isn't what you're complaining about.
In my opinion gaming isn't a little ego-fest where the GM displays his prize creation for the players to adore; it's a communal, highly collaborative experience where everyone is supposed to have fun. Heck, I prefer roleplaying to combat, but I'll surely run a combat heavy game if my players prefer; doing otherwise would be akin to me serving only meat to a vegetarian who I invited over for dinner, just because I like meat myself.
And in my opinion, gaming isn't a little ego-fest where the GM panders completely to the players. Of course, you're not saying that, but that's the opposite of what you're saying...and neither is what anybody here is actuallly saying as far as I can tell. The truth is somewhere in the middle. Sometimes the GM does have to do things that one or more players may not want, and that's not a bad thing. Sometimes it is. Blanket statements though, won't get us much of anywhere in this discussion.